The invention relates to a method for welding a bolt onto a work piece by means of a bolt-welding device having at least one drive shaft connected to the bolt to be welded and at least one electromagnetic linear drive acting in axial direction with at least one coil.
A weld-on method is known from DE 43 24 223 A1, for example. In this case, two electromagnetic driving devices acting in opposite directions are provided, one acting in forward direction and the other in the opposite direction. If the drive shaft has to be repositioned, the drive acting in forward direction, which is the stronger anyway, will apply such a force to the drive shaft as to cause a displacement of the drive shaft, while the other linear drive also acts during the displacement. Because the two linear drives act against each other, a balance of forces is required for fixing a position, but this means that considerable electric energy must be available, which leads to an increase in the heat development, especially when the drive shaft is in slanted position.
Therefore, it is suggested by DE 195 29 350 to always activate only one of the two linear drives at any one time. Using this method, it is difficult to quickly and securely move the drive shaft to intermediate positions between the starting position (drive shaft fully retracted) and the end position (drive shaft fully extended). It causes a so-called transient behavior of the drive shaft around the target position. Moreover, the drive shaft may also oscillate in certain welding positions (trough or overhead positions).
The invention provides a method and a device, especially for performing the method, which are simple and ensure that the drive shaft is positioned quickly and precisely. Moreover, few heat problems will be encountered with the method of the invention and with the device of the invention. Also, the reproducibility of the welding process is improved for all welding positions.
According to the method of the invention, when the drive shaft is positioned a selectively switchable force is applied to the drive shaft by means of creating a structural path of flux between the drive shaft and the housing, where the force exerts a resistance against displacing the drive shaft, at least in forward direction. According to the method of the invention, the housing and the drive shaft can be structurally coupled directly or indirectly, i.e. mechanically, and a force can be applied to the drive shaft via said coupling which exerts a resistance against displacing the drive shaft, at least in forward direction. This clearly sets the invention apart from the methods intending to position the drive shaft by means of two electromagnetic linear drives acting in opposite direction and from the methods where a spring permanently applies a restoring force. According to the invention, the resistance is used to position the drive shaft or to facilitate positioning. For example, the resistance can dampen the movement to a target position so that the so-called transient oscillation into the target position is reduced or fully prevented. By means of the switchable force, the drive shaft can be securely fixed in position even in the retracted starting position, i.e. the electromagnetic drive system can be switched to an approximately or completely currentless state, which, in turn, reduces the heat development.
Moreover, the drive shaft can be held in any other position, specifically in intermediate positions between the starting and end positions, by means of the switchable force.
In said intermediate positions, the electromagnetic linear drive can then also be currentless.
The level of the force is preferably variable. Because the force is adjustable, it is possible to selectively exert a resistance against the displacement of the drive shaft so as to dampen its movement or prevent movement, which means that the drive shaft is securely fixed in position by means of said force.
The gravity of the drive shaft and the movable parts coupled to the drive shaft can be partially or fully compensated by means of said force. When working overhead, for example, i.e. when the drive shaft is directed upward with the bolt to be welded in front, the force prevents the drive shaft from sliding backward and downward as a result of its own weight. When the drive shaft is directed downward with the bolt in front, such a force can be applied to the drive shaft against the forward direction that said force compensates at least partially or even fully the displacement force caused by the weight of the drive shaft and the parts that are moved together with the drive shaft. Therefore, the electromagnetic linear drive has to consume less electric power.
As explained above, the linear drive should preferably be supplied with less electric energy when the force is connected than when the force is not connected so as to reduce the required driving energy.
The force can be a clamping force, for example, applied between the drive shaft and a part mounted to the housing, which includes the housing itself. For example, a hydraulic, pneumatic or even an electric drive, for example in the form of a lifting magnet, is suitable.
The force can be directed transverse to or opposite the forward direction. In the latter case, the force could also define a restoring force to a starting position.
According to an embodiment of the invention, the force is apportioned relative to the forward force of the linear drive such that it has a dampening function when the drive shaft is moved to a target position.
In addition, the invention relates to a bolt-welding device, especially for performing the method of the invention, having at least one drive shaft connected to the bolt to be welded, an electromagnetic linear drive acting in axial direction with at least one coil, where the drive shaft is part of the linear drive. The bolt-welding device of the invention is characterized in that a switchable device is provided on the drive shaft or the housing and engaging on the housing or the drive shaft when connected, applying a force to the drive shaft and exerting a resistance against a displacement of the drive shaft, at least in forward direction.
As explained above in relation to the method, the device can preferably change the level of the resistance.
According to an embodiment of the invention, said device is a clamping device acting transversely to the drive shaft, for example in the form of a hydraulic or pneumatic piston-cylinder drive.
When the piston-cylinder drive is coupled with a valve which is able to switch the cylinder to a depressurized state, then the drive cannot exert a resistance or it can exert only a very marginal resistance against displacement.
According to another embodiment of the invention, a position measuring system and a control system are provided where the control system is coupled to the position measuring system and is able to drive the device. The position measuring system can determine whether the drive shaft is slanted relative to the horizontal plane. The position measuring system has a double function in this case. As soon as a slanted position is detected the device can be controlled such that it counteracts a displacement of the shaft and/or compensates the effect of the weight of the shaft.